William Eggleston is the man credited with making colour photography into an acceptable art form. It happened as late as 1976 with an exhibition at MoMA, New York when art photography, if you recognised such a genre, still meant black-and-white. Eggleston’s pictures showed nothing dramatic, just unspectacular, everyday scenes; images from a man committed, as he said, simply to “photographing life”. Henri Cartier-Bresson proclaimed that photography was all about capturing “the decisive moment” and for him that meant instances of oddity or compositional wit. Eggleston, born in 1939, is a devotee of Cartier-Bresson but his decisive moments lie in the mundanity of a man eating a burger, a lightbulb against a red ceiling, a girl in a bar drinking from a bottle of beer or an elderly woman in a floral dress smoking a cigarette.

His realm is not the great urban centres of America but the old-fashioned states of the south where he grew up and lives — Georgia, Mississippi and Tennessee: ordinary regions for ordinary subjects. Except that his photographs are not that ordinary, as the exhibition of 100 of his portraits at the National Portrait Gallery reveals. In their very nondescriptness they show that there is interest to be found in every face and every object — that everything is worth a second glance because there is an inherent strangeness to it.

The artist Ed Ruscha, another aficionado of the gas station and diner, said of Eggleston: “When you see a picture he’s taken, you’re stepping into some kind of jagged world that seems like Eggleston World,” but he is only half right. The photographs may be distinctively Eggleston’s but the world they reveal is an Everyman’s America. He adds no props and photographs what he sees without rearranging or posing his subjects. He photographs the banal without taking banal photographs.

This exhibition mixes his early black-and-white portraits with his later colour ones and his subjects are a mix too of anonymous sitters, of friends and family and of the famous. Dennis Hopper takes his place alongside snatched images of a young black woman leaving a burger bar or a boy caught in the low evening sun pushing a row of supermarket trolleys. It was this last image, taken in 1965, that Eggleston regards as his first successful colour photograph (see below). It was, he said, just a picture of “some kind of pimply, freckle-faced guy in the late sunlight. And by God, it all worked.”

What makes his pictures so striking is their colour and their sense of mystery. Eggleston started working properly with colour transparency film in the mid 1960s but it wasn’t until the early 1970s when he came across the dye-transfer process usually used for graphic art that he found a way of fixing the saturated tones he saw around him: “Every photograph I subsequently printed with the process seemed fantastic and each one seemed better than the previous one.” His reds became richer, his blues more blue, his yellows sharper.